To locate an object at a distance, particularly when the object is relatively small and the distance is relatively large, is difficult and sometimes is impossible or nearly impossible. For example, prompt locating of a survivor of a ship wreck in a vast body of water usually is critical, and if unsuccessful can lead to tragedy. Radios have been used in the past to signal the location of such a survivor; but radios are relatively expensive, complex and power consuming and may be somewhat ineffective in the presence of substantial wave action and/or other background noise.
A closed communication system is one in which information can be communicated in one or sometimes two directions without or with minimal possibility of that information being detected by an unauthorized individual.
Electromagnetic energy, as it is used herein, means such energy that is capable of being reflected, preferably by means of a retroreflector device. An example of electromagnetic energy includes light in the visible spectrum, infrared energy/radiation, and ultraviolet energy/radiation. The invention will be described hereinafter with respect to light in the visible spectrum; however, it is intended and it is to be understood that the invention encompasses use with electromagnetic energy that is other than visible light, as long as such electromagnetic energy is capable of reflection, more preferably, retroreflection.
In the field of optics retroreflectors are well known. The function of a retroreflector is to receive input electromagnetic energy and to reflect such energy back from whence it came along the same path it was received and preferably with the same general characteristics, relatively high gain and relatively little spreading of the light beam. One exemplary retroreflector is a corner reflector; another is one that uses plural spheres and curved reflectors associated therewith.
Shutters for controlling transmission of light also are known. One example is a mechanical shutter used to control light input to a camera. Another type of shutter is a liquid crystal shutter. Exemplary liquid crystal shutters are disclosed in U.S. Pat. Nos. 4,435,047, 4,556,289 and 4,606,611. Still another type of liquid crystal shutter is described in U.S. Pat. Nos. 4,385,806, 4,540,243, and Re32,521, which concern on surface mode switching and optical polarization phenomena. The entire disclosures of the patents mentioned herein are hereby incorporated by reference, and it will be appreciated that the shutters disclosed therein may be used in the context of the present invention. Other types of liquid crystal shutters also are known; examples include twisted nematic liquid crystal shutters and dynamic scattering liquid crystal devices.
Since the invention is disclosed in detail below with reference to light, such shutters may be referred to below as optical shutters or light shutters, although it will be appreciated that shutters used in accordance with the present invention broadly are those intended to control transmission of electromagnetic energy and would be selected according to the nature of the electromagnetic energy and the operating requirements, such as speed of response, opaqueness, etc. required for a particular system. Moreover, although according to the present invention a liquid crystal shutter is preferred to modulate light, particularly reflection of light, as is described further below, it will be appreciated that other types of shutters also may be used consistent with the invention.
Above-mentioned U.S. Pat. No. Re32,521 discloses a communications transmitting and receiving installation. At the receiving installation there is a light source, which directs light to the transmitting location, and a light demodulator. At the transmitting location the received incident light beam is modulated and is reflected back toward the demodulator of the receiving installation for detection of the modulation information. Surface mode switching of liquid crystal orientation and, thus, polarization effects are disclosed in such patent for use to impose information on the transmitted light for subsequent detection at a receiver. In one embodiment disclosed in such patent a corner reflector is used to reflect incident light to a detector. In a sense the mentioned apparatus at the transmitting location disclosed in such patent is passive in that the light source providing energy therefor need not be a part of the transmitter; rather, the transmitter receives light from a remote source and modulates and reflects the light for subsequent detection. One disadvantage to the system of the '521 patent is the need to detect polarized light, for when light travels long distances through the atmosphere substantial depolarization and, thus, signal degradation, can occur as a result of gases, particulates, dirt, etc. in the optical path.